Our intention is to continue our investigations of the ileal and renal bile salt active transport systems which were first described by this laboratory. These studies will progress along the following lines: a) Further delineations of the mechanism of the active ileal transport systems. b) The physiological and pharmacological effects that inhibitors of these systems may produce. c) The clinical implications arising from the pathology of the G.I. tract with emphasis on pathologies of the ileum. In addition our work during the past five years has been extended to active transport in the liver and metabolic events associated with the biosynthesis of bile salts from cholesterol. Our intentions in these are to: a) Continue our studies of the role of cytochrome P450 in the rate limiting step of bile salt biosynthesis (7 alpha-cholesterol hydroxylase). b) Continue our studies of our findings of the inverse relationship between enhanced bile salt biosynthesis and decreased levels of hepatic drug metabolizing enzymes. BIBLIOGRAPHIC REFERENCES: Johnson, D.B., M.P. Tyor and L. Lack. The Use of (7 alpha 3H) and (7 alpha, 7 beta 3H)-Cholesterol in the Enzymic Assay of Cholesterol 7-Hydroxylase. J. Lipid Res. 17:353, 1976. Gallagher, K., J. Mauskopf, J.T. Walker and L. Lack. Ionic Requirements of the Active Ilial Bile Salt Transport System. J. Lipid Res. 17:572, 1976.